The present invention relates to roller worm drives for mechanical power transmission and to roller worm wheels for use in such drives.
The conventional worm gear drive includes a worm screw and a wheel having teeth shaped to conform to the threads of the screw. As the worm screw rotates, its threads engage the teeth on the wheel and drive the wheel in rotation. With conventional worm gear drives, there is sliding motion, and hence considerable friction, between the surfaces of the screw threads and the surfaces of the teeth on the wheel. Such sliding friction results in considerable power loss.
As disclosed in U.S. Pat. No. 626,515, issued June 6, 1899, these frictional losses can be substantially eliminated if the wheel is provided with rollers rather than teeth. As taught by the '515 patent, the rollers may be arranged so that the axis of each roller extends substantially radially with respect to the axis of the worm wheel, and each roller may be mounted to the worm wheel by means of anti-friction bearings such as ball bearings. Each roller has a tip projecting outwardly from the body of the wheel. As the worm screw rotates, the screw thread surfaces engage the tips of successive rollers and drive the wheel in rotation. Because each roller is free to rotate its own axis about relative to the worm wheel, there is substantially no sliding friction between the roller tips and the screw thread surfaces.
Despite the manifest advantages afforded by such roller worm drives in eliminating sliding friction, these drives have not been widely adopted heretofore. Roller worm drives heretofore have not provided power transmission capacity competitive with the capacity of conventional worm drives of comparable size. The power transmission capacity of a roller worm drive ordinarily is limited by the load carrying capacity of the anti-friction bearings connecting the rollers with the body of the wheel. The size and hence the load carrying capacity of the bearings is limited by the space available in the wheel for mounting the bearings.
Notably, roller worm drives having a single set of radially arranged rollers heretofore have been unsuitable for use in automotive final drive systems. It has been impracticable heretofore to provide such roller worm drives with the required power transmission capacity without making the roller worm drive so large as to exceed the available space in the vehicle.
One useful solution to this problem is taught by U.S. Pat. No. 4,685,346 and by the corresponding International Patent Publication WO 85/04228, published Sept. 26, 1985. As set forth therein, two sets of angularly arranged rollers may be provided rather than a single set of radially arranged rollers. The angularly arranged rollers are disposed so that the rollers of one set project in one direction along the axis of the worm wheel, whereas the rollers of the other set project in the opposite direction along the axis of the worm wheel. The rollers of the two sets are staggered, so that each roller of one set is disposed between rollers of the other set. Because the rollers are angularly arranged in this fashion, the anti-friction bearings mounting each roller to the worm wheel are spaced along the roller axis from the anti-friction bearings mounting the immediately adjacent rollers to the body of the worm wheel. This arrangement substantially increases the space available for the anti-friction bearings in a wheel of given size, and hence substantially increases the power transmission capacity which may be provided with a roller wheel of given diameter.
As also disclosed in the '228 Publication each roller may be mounted for sliding movement over a limited range along its own axis relative to the body of the wheel. Biasing springs can be provided for urging the roller outwardly with respect to the wheel, into engagement with the thread of the screw. With this arrangement, any individual roller which may be overloaded will be forced inwardly relative to the body of the wheel. This arrangement thus provides for equal sharing of transmitted loads between a plurality of rollers simultaneously in mesh with the threads of the screw despite any minor inaccuracies in the device, and hence provides still further improvements in load carrying capacity. However, in the construction disclosed by the '228 Publication, the biasing springs, the thrust bearings associated therewith and the retaining rings employed to limit outward movement of the rollers all consume appreciable space within the roller wheel. These additional elements thus undesirably reduce the space available for the bearings.
Thus, although worm drive systems according to the '228 Publication provide significant improvements in roller worm drives, there has been need for further improvement.